Tire testing systems for determining the presence of irregularities or non-uniformities in tires are known in the art. Conventional systems typically move a tire to a testing station where it is engaged by some form of chuck assembly and inflated to its normal pressure. The tire is rotated at its normal speed and contacted by a loadwheel which is free to rotate about an axis parallel to the rotational axis of the tire. The loadwheel has spindles at its opposite ends provided with load cells which measure forces acting on the loadwheel in directions of interest. Precise measurement of the forces exerted by the tire permits accurate adjustment of the uniformity of the tire after the force measuring procedure, for example, by grinding devices which remove excess tire material to correct any irregularities that may have arisen during the manufacturing process.
Conventional tire testing systems such as that described above are disclosed in, for example, U.S. Pat. Nos. 4,704,900 and 4,805,125. In these systems, the loadwheel spindles are provided with load cells and secured to a movable carriage. The carriage is attached to a ball screw housed in a screw shaft, the screw shaft being rotated by a chain-sprocket assembly driven by a motor and gear reduction unit. Rotation of the screw shaft moves the ball screw and carriage toward or away from the tire being tested, the carriage sliding along the frame of the machine. A servomechanism moves the carriage to a desired position based on the force signals generated by the load cells.
Although prior art tire testing systems, and in particular known loadwheel assemblies used therewith, measure tire uniformity in an acceptable manner, several drawbacks exist so as to leave room for improvement. In conventional systems the loadwheel is rotatably mounted on a C-shaped carriage such that the rotational axis of the loadwheel passes through the two horizontal legs of the "C." The end portion of each horizontal leg of the carriage is provided with mounting structure for attachment of the loadwheel spindles. More particularly, the spindles are attached to the side of each horizontal leg of the carriage. As such, the load cells carried by the spindles are only accessible from one side of the carriage legs. Consequently, the load cells can be accessed only from one side of the frame of the testing machine because the horizontal carriage legs prevent access from the other side of the frame.
In view of the fact that the load cells have a limited useful life, they must at times be replaced to ensure proper operation of the machine. The limited access to load cells in prior art machines poses an obstacle to replacement should access from the one side of the machine frame be difficult or unavailable, for example, due to intervening structure of the testing system or the environment in which the machine is located. Accordingly, there is a need in the art for an improved loadwheel assembly that allows easy replacement of the load cells despite the specific layout of the machine.
Another drawback affecting operation of prior art loadwheel assemblies relates to the structure of the loadwheel. Specifically, conventional loadwheels comprise a cylindrical wall (the outer surface of which contacts the rotating tire) and a hub connected to the wall by a plurality of plates or spoke-like ribs. For example, one commonly used loadwheel includes twelve ribs extending between the outer wall and the hub. Such construction has sometimes resulted in inaccurate measurement of the force exerted on the loadwheel by the tire. Further, a loadwheel having a plurality of ribs may experience harmonic related problems during use. In addition, prior art loadwheels like those discussed above typically are formed by an expensive casting process, e.g. magnesium casting, which increases the cost of the testing machine.
Further, in conventional loadwheel assemblies the carriage is moved toward and away from the rotating tire by a motor and gearbox mounted to the machine frame. A sprocket and chain assembly mounted to the frame transfers the motor output to a screw assembly to move the carriage and loadwheel. Keeping in mind that tire testing machines are relatively complicated apparatus, the sprocket and chain assembly constitutes an additional component requiring maintenance and increases the susceptibility of the machine to breakdown. As such, provision of the sprocket assembly increases the likelihood of machine downtime which adversely affects tire testing efficiency.
Accordingly, there is a need in the art for an improved loadwheel assembly that overcomes limitations of conventional tire testing machines, provides increased flexibility in maintaining and servicing the machine, simplifies the overall construction of the machine, and accurately measures forces generated by the tire.